The Optical Counterpart to the Accreting Millisecond X-Ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440

We used a combination of deep optical and $H\alpha$ images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021 during quiescence. A strong $H\alpha$ emission has been detected from a main-sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.″15 from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of $0.70\text{--}0.83M_{\odot }$, a radius of $0.88\pm 0.02R_{\odot }$, and a superficial temperature of 5250 ± 80 K. These parameters, combined with the orbital characteristics of the binary, suggest that the system is observed at a very low inclination angle ($\sim 8^{\circ }\text{--}{14}^{\circ }$) and that the star is filling or even overflowing its Roche lobe. This, together with the EW of the $H\alpha$ emission (∼20 Å), suggests possible ongoing mass transfer. The possible presence of such an ongoing mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.